Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52

We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608-52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks, 30 and 18 seconds after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608-52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.Comment: Accepted for publication in the Astrophysical Journa

An elog-KdVB dynamics in non-thermal solar plasmas

This paper deals with a continued study on the basis of the gravito-electrostatic sheath (GES) model to explore the excitation of solitary and shock-like wave structures evolving in the non-thermal solar plasmas. The method applied here is based on a nonlinear local perturbation analysis over the GES structure equations designed in a thermostatistically modified form to arrive at an extended logatropic Korteweg-de Vries-Burgers (elog-KdVB) equation with a unique linear derivative source, which has in principle, a special set of multiparametric coefficients dependent on the diversified solar plasma parameters. A constructive numerical integration of the elog-KdVB equation yields the excitation of rarefactive shock-like wave patterns supported in the solar plasmas. Their noticeable unique characteristic feature is the naturalistic existence of distorted non-uniform tails. The shock-wave amplitude increases with the increase in the thermostatistical power (κ), and vice versa. In contrast, the shock-tail width decreases with the increase in the thermostatistical distribution power (κ), and vice versa. It implicates that the shock-tail width vanishes in the Boltzmann thermostatistical limit ($\kappa \to \infty$ ). The corresponding gradients, phase portraits, and curvature dynamics associated with the fluctuations are illustratively depicted. The microphysical details behind the dynamics are analyzed. The elog-KdVB dynamical results explored are bolstered with the reinforcement of the earlier multispace satellitic observations and original probe measurements reported elsewhere. The non-trivial implications and applications are summarily highlighted in the real helioseismic contextual linkage

Burst–Disk Interaction in 4U 1636–536 as Observed by NICER

International audienceWe present the detection of 51 thermonuclear X-ray bursts observed from 4U 1636–536 by the Neutron Star Interior Composition Explorer (NICER) over the course of a 3 yr monitoring campaign. We perform time-resolved spectroscopy for 40 of these bursts and show the existence of a strong soft excess in all the burst spectra. The excess emission can be characterized by the use of a scaling factor (the f$_{a}$ method) to the persistent emission of the source, which is attributed to the increased mass accretion rate onto the neutron star due to Poynting–Robertson drag. The soft excess emission can also be characterized by the use of a model taking into account the reflection of the burst emission off the accretion disk. We also present time-resolved spectral analysis of five X-ray bursts simultaneously observed by NICER and AstroSat, which confirm the main results with even greater precision. Finally, we present evidence for Compton cooling using seven X-ray bursts observed contemporaneously with NuSTAR, by means of a correlated decrease in the hard X-ray lightcurve of 4U 1636–536 as the bursts start